Swift 18 Owners

Can anyone recommend a supplier or maker of Cabin seat covers and foam inserts?

Jeckells Upholsterers of Wroxham, Norfolk might be worth a look.

If you have the originals, for taking a pattern and, maybe reusing the wooden infills, any competent upholsterer should be able to do this job, it is fairly straightforward. Marine/boat usually costs more and the materials are not 'marine' when all is said and done.
I'd avoid all leatherette cushions BTW, they are miserable for sleeping on.